The present invention relates to papermaking, and more particularly, to papermaking and products made thereby with an ionic crosslinked polymeric microparticle-containing retention aid system.
In the production of paper and paperboard from a dilute aqueous suspension of cellulose fibers on papermaking apparatus, the suspension can be passed through one or more shear stages and the resultant suspension is drained through a wire to form a sheet, which is then dried. It is common to include various inorganic additives and/or organic polymeric materials in the suspensions in efforts to improve the process or products thereof. Process improvements have been sought in the past, for example, in retention, drainage, and drying, and in the formation (or structure) properties of the final paper sheet. However, these or other papermaking parameters can come into conflict with each other in unpredictable manners. Retention is a term used in papermaking to indicate the extent to which the pulp fibers and other additives which are added to the furnish are retained in the finished paper. A retention aid generally acts by increasing the flocculating tendency of the pulp fibers and additives to inhibit their loss during drainage through the paper machine wires or screens. Drainage or de-watering is another papermaking requirement, which tends to conflict with retention, such as when a rapid reduction is sought in water content of an aqueous pulp suspension in the sheet forming areas of a paper machine.
Microparticles and other particulate materials have been added to papermaking pulps as retention aids. Different from water-soluble polymer retention systems, microparticle retention systems typically refer to water-insoluble microparticulate retention aids. It is known to use silica, silica sols, and bentonite, as inorganic or mineral microparticles for retention aids in papermaking. Specific polymeric microparticles have been used as retention aids in papermaking. For example, U.S. Patent Application Publication No. 2006/0142430 A1 relates to a method which provides for addition of a composition that includes an associative polymer and an organic microparticle, which microparticles are copolymers of a nonionic monomer, an ionic monomer, and a crosslinking agent, and optionally a siliceous material to a papermaking slurry. U.S. Pat. No. 5,171,808 relates to crosslinked anionic and amphoteric microparticles.
Other chemistries and forms of organic microparticles, different from previous microparticles, would be useful in improving wet-end drainage and retention in papermaking.